Last week, Apple updated its iMac 21.5-inch and iMac 27-inch desktops, adding Thunderbolt ports, better graphics cards and more. Perhaps the most interesting change to the iMac line, however, was the inclusion of quad-core Intel processors with every iMac configuration, from the entry-level model on up.

That, combined with the really good screen the iMac 21.5-inch already had, means that Apple is now offering - after a multi-year drought - a desktop Mac that combines truly impressive processing power, a colour accurate display and a wealth of other useful features into a single computer - at a bargain price.

We review the new iMac 21.5-inch, with an emphasis on its display and processor and how they've teamed up to make the computer an enticing option for photographers on a budget.

A brief history of the iMac 20-inch

To understand our excitement for the new iMac 21.5-inch, you have to step back back in time, to 2006. Early that year, Apple produced a desktop computer that was both economical and stuffed with features for photographers. It was the iMac 20-inch, one of the first models from the company to incorporate Intel processors, and its newfound speediness was a signature attribute of the all-white, all-in-one machine, relative to the PowerPC processor-equipped model it replaced.

Better performance was only part of what made the computer a good fit for digital SLR shooters. Just as compelling was its colour accurate IPS-technology display, which profiled well and was every bit as good as Apple's impressive (and pricey) Cinema Display 20-inch of the day. The iMac 20-inch also had a full complement of USB and FireWire ports, Gigabit Ethernet, Wi-Fi, Bluetooth, a built-in iSight video camera and it could drive an external monitor too.

For less than US$1700 when it launched, the computer was a true bargain for the photographer who wanted decent performance, a good quality screen on which to assess and adjust pictures, fast wired networking and enough versatility to handle regular computing tasks also. Its utility has been enduring: The 2006-vintage iMac 20-inch I purchased is still in daily use here, while site co-editor Mike Sturk just last month replaced the one he owned. This computer was a gem.

Apple phased out the white iMac 20-inch in the middle of 2007, and phased in an aluminum-clad replacement whose display was no match for its predecessor. Poor screen evenness and inaccurate colour, even when profiled with a hardware calibration device, meant the short, golden period of the iMac 20-inch was over.

Apple improved the display as part of this model's transition into the iMac 21.5-inch in 2009. At the same time as they added 1.5 inches to the screen, Apple also rolled out quad-core processors to the iMac line. The catch? To get a quad-core processor you had to move up to the new iMac 27-inch, introduced alongside the iMac 21.5-inch in October of that year. So, the smaller and more affordable of the two iMac models, while no longer sporting a hobbled display, was now not capable of nearly the same processing performance as its larger-screened counterpart. This meant it was not yet time for the entry-level iMac to return to its former glory.

The quad-core iMac 21.5-inch arrives

All that changed last week, when Apple at last reintroduced the mix of speed, display quality, versatility and low cost that made the 2006 iMac 20-inch such an excellent choice for shooters on the Mac platform. It took five years and four months, but as you'll see in our testing, it was worth the wait: the "Mid-2011" model of the iMac 21.5-inch has nearly all that a value-minded photographer could hope for in a Mac desktop that starts at under US$1200 in the U.S.

Budget Power: The quad-core Apple iMac 21.5-inch, introduced last week for US$1199 in the U.S. Click to enlarge (Photo by Rob Galbraith/Little Guy Media)

But wait, there's more. Apple included a dual-purpose Thunderbolt port too. It enables an external monitor to be connected, or for the iMac 21.5-inch's display to serve as an external monitor to other Macs with Thunderbolt ports. The same port can connect to high-speed storage as well, or at least it will once the first peripherals with Thunderbolt ports ship later this year. The iMac 21.5-inch can be outfitted internally with a 3.5-inch hard drive, 2.5-inch SSD, or both, and with up to 16GB of RAM (in four easy-access slots.)

Note: A detailed description of the new iMac's ports and related features is here. Detailed specifications are here.

The entry-level configuration, which includes a Core i5 2.5GHz quad-core processor, 500GB/7200rpm hard drive and 4GB RAM, is US$1199 in the U.S. Another US$50 in reliable aftermarket RAM brings the machine up to a more practical 8GB, and the total cost to about US$1250.

US$1250 is only a sweet deal if the iMac 21.5-inch's speed and display quality are as potent in reality as on paper. As we've already declared, the news in both areas is good, even great.

Display quality

The 21.5-inch (diagonal) flat panel display in the iMac 21.5-inch has a resolution of 1920 x 1080 pixels. It employs LED backlighting and, like most of Apple's product line, is available only in a glossy version (which means it's usable for image editing only under conditions in which reflections can be controlled). A teardown reveals that it's manufactured by LG Display.

There are three main types of display technology used in computer screens these days:

Twisted Nematic (TN)

Vertical Alignment (VA) and its most common subtype, Patterned Vertical Alignment (PVA)

In-Plane Switching (IPS)

Of these, IPS generally offers the widest true-colour viewing angles and tends to respond better than other display technologies to profiling with a colorimeter or spectrophotometer-based monitor calibration package.

There are exceptions, and even within the realm of IPS displays there is a range of quality and achievable colour accuracy, not to mention several IPS technology variants. But, in general, an IPS display will be better for viewing and correcting photographs than one which utilizes a VA variant or TN (especially TN). IPS is what's inside the iMac 21.5-inch.

And then compared the various profiled results to two reference monitors, an Apple LED Cinema Display 24-inch and NEC MultiSync PA241W. All analysis was done the old-fashioned way, by carefully looking at colour and black and white photographs from a batch of nearly 150 that we've gathered over time to evaluate monitors and printers. Plus colour and gray ramps, gray wedges and the like.

We'll stop short of giving a blow-by-blow of the entire profiling and evaluation process and instead get right to the point. The display in the iMac 21.5-inch delivers very, very good colour accuracy and reasonably neutral grays within its gamut, which approximates sRGB. Difficult reds and oranges, deep blues and magenta, memory colours like green grass, they all fall into place as they should.

Display shortcomings are mostly minor. For example, brightness is very slightly uneven across the screen area, plus small changes in viewing angle introduce very slight shifts in skin tone rendering.

The only display trait that could be troublesome is a tendency to plug up slightly in deep shadows. When the active profile was generated by i1Profiler or Spyder3Elite, there was no visible difference between level 0 (pure black) and level 2; with a profile from Display Pro, level 0 to level 5 were indistinguishable. On a 0-255 scale, a lack of differentiation in the bottom handful of levels may not seem significant, and it isn't. But it does give the appearance of slightly too-high contrast and a lack of very dark shadow detail in pictures that do in fact have both pleasing contrast and detail just above pure black.

By comparison, the same profiling software and devices, all of them, combined to make profiles for the LED Cinema Display 24-inch that show a differentiation from level 0 to level 1 and up from there.

All in all, however, the iMac 21.5-inch's display is really very good, and fantastic when you factor in the cost of the computer it comes in.

Note: As with any computer screen from Apple we've tested in the past several years, whether it's in an iMac, external monitor or laptop, the canned profile automatically assigned to it by the operating system is poor. Pictures look much more contrasty and harsh than they actually are, while certain colours, such as reds and yellows and warm skin tones, are unrealistically oversaturated. To unlock the full quality of the iMac 21.5-inch's display, a custom profile created by a decent third party package is mandatory.

Processor performance

An Intel quad-core processor is at the heart of every iMac 21.5-inch now. Either the Core i5, running at 2.5GHz or 2.7GHz, or the Core i7 operating at 2.8GHz. Regardless of the processor option you select, it will have four cores available to convert RAW files, export video or whatever else you do with a computer that requires CPU muscle.

The table below puts some numbers to the experience of moving from a fairly modern two-core Mac to a new four-core Mac: the four-core computer feels at least twice as fast. The two Macs we tested were:

The source and destination in each case was an external FireWire 800 drive, to help ensure the comparison was of the processors and not drive speed. The operating system version was Mac OS X 10.6.7. For the RAW conversions, noise reduction and sharpening were applied during the conversion. For the video transcoding test, a 10:38 long, 1080p/30fps clip was converted with the iPad preset selected in Handbrake.

The gains shown are likely be representative of what you'll experience across the board, when the iMac 21.5-inch is given a processor-intensive task. It wasn't so long ago that as many as three cores in a four-core Mac could be found sitting idle for long stretches, even while the active program was working through a heavy duty operation like converting digital SLR RAW files. Now, it can be hard to find pro-level photo or video software for Mac that doesn't light up every core when there's serious work to be done.

Changes in both the Mac OS, and in the various programs themselves, have led to the improved use of multiple processing cores. Earlier versions of Capture NX2, for example, made limited use of more than one processing core, and the program was often infuriating slow as a result.

Not anymore: Capture NX2 v2.2.7, running in OS X 10.6.7, calls upon four cores at every turn, not just when batch processing (as shown in the screenshot below) but when applying almost any kind of adjustment the program offers. Capture NX2 is still no speed demon, but by taking better advantage of multiple processing cores it no longer feels like it's towing a piano, even as it juggles 16.08 million pixel D7000 NEFs.

The benefits of four cores extend to more than just converting RAW data and video. If, for example, you depend on Camera Bits Photo Mechanic for Mac to browse your way through a contact sheet of high-resolution JPEGs, you can expect a palpable speed boost in both full screen and full resolution rendering when you step up from two cores to four.

This program has long made excellent use of multiple processing cores, a fact that's fully evident when previewing and zooming hundreds of 5D Mark II and Nikon D3X files in a session on the iMac 21.5-inch. Not only does Photo Mechanic extract full benefit from the computer's processing power, it also demonstrates how much faster it is at displaying full resolution JPEGs than any other Mac program we've ever tried, including Lightroom and Apple Aperture. With four cores at its disposal, Photo Mechanic's speed advantage only becomes that much more obvious.

The 2 x 50 test of Digital Photo Professional (DPP) reveals something interesting. As we've already established, DPP, Capture NX2 and Lightroom for Mac programs will draw on all four cores during RAW conversion. But, none are designed to maximize the full potential of this many processing cores.

Converting multiple pictures simultaneously during a batch conversion is one example of how they could, but this isn't an option per se in any RAW converter we've tested. That said, this exact behaviour can be faked in DPP. By separating a large group of CR2s into two smaller ones, then starting each group as its own batch, DPP - or rather its companion program, Digital Photo Professional Batch - will convert two pictures at the same time. It will also nearly saturate the iMac 21.5-inch's four cores as it does so.

If you compare DPP's performance in the one-batch-of-100 and the two-batches-of-50 tests, the latter shaves 30% off the total processing time required on the iMac 21.5-inch. The two-core MacBook Pro also benefits from this trick, though only about half as much, in percentage terms, as the four-core iMac. Also note that, on the iMac, total processing time shortens even further if three DPP batches are run concurrently. The improvement is only incremental, however.

A 2010 MacBook Pro 17-inch or 2010 iMac 21.5-inch would have completed each test a bit faster than the 2009 MacBook Pro we had representing the dual-core camp. But the outcome would have been the same: four processing cores drastically reduce the time it takes to finish intensive jobs, like converting RAW files, exporting digital SLR movies or browsing large collections of JPEGs. It's hard to beat the benefit of more cores these days, and it makes the iMac 21.5-inch not only quicker than a comparable two-core machine, it also makes it just plain fast. The only way to lay your hands on a way faster Mac is to opt for a 6, 8 or 12-core Mac Pro for several times the price.

Before we leave the topic of performance, note that the iMac 21.5-inch's FireWire 800 and USB 2.0 ports operate as fast as any other Mac, at about 93MB/s and 36MB/s in sustained large file transfers, respectively.

Great, but not flawless

So far, this has been an iMac 21.5-inch lovefest. The computer is really nice, but it isn't without flaws.

No USB 3.0 Apple has so far chosen not to include USB 3.0 ports on its computers, and the iMac 21.5-inch is no exception. As both USB 3.0 drives and USB 3.0 memory card readers emerge, we're ready to see USB 3.0 ports replaces USB 2.0 everywhere, including on Apple's all-in-one desktops.

In time, Thunderbolt drives and readers may at least partly supplant the need for USB 3.0, and maybe we'll also see a Thunderbolt-to-USB 3.0 adapter. But, it's a shame to have a row of four USB ports on the back of the iMac 21.5-inch that are all limited to USB 2.0's 36MB/s or so, when hard drives, SSDs, CompactFlash and SDHC/SDXC cards are capable of much faster read and write speeds than this.

Hard drive is difficult to replace The iMac 21.5-inch can be ordered with a hard drive and/or SSD drive inside, which is great, as is the fact the SATA connections to both are the latest and fastest variants of this type, called SATA 6 Gb/s (and commonly referred to as SATA 3).

Several years down the road, however, when it's time to replace either one, you probably won't be able to do it yourself. The difficulty of taking apart the computer and the likelihood that you'll see trapped dust and fingerprints on the inside of the display's cover glass when you've put it back together (if you get it back together!), these are reasons enough to keep out of the inside of this Mac.

There's an additional wrinkle that applies to the 3.5 hard drive only: because of a new and apparently proprietary thermal sensor cable used in the new model, which may be tied to custom firmware in the drive as well, you'll almost certainly need Apple to perform the replacement and put in only a drive that's specifically compatible with the thermal sensor arrangement. Even if you have the tools and skills to take apart and reassemble the iMac 21.5-inch properly.

It has always been difficult to replace a hard drive in an Intel processor-equipped iMac, so the main thrust of what we're reporting isn't new. It's just something to be aware of, before you buy.

The same is NOT true for RAM: the computer's four slots are easy to get into, while any decent aftermarket RAM that meets the machine's RAM specs should work.

External monitor bug? We tried connecting an Apple LED Cinema Display 24-inch to the Thunderbolt port of the iMac 21.5-inch. The external monitor was recognized properly and, thanks to sufficiently powerful graphics circuitry in the computer, it could play a lengthy clip of unedited 5D Mark II 1080p/30fps video on both screens simultaneously without hiccups or dropped frames. Plus, the iMac 21.5-inch was able to use the mic, speakers and iSight video camera built into the external display (not that you'd need to very often, but it can be done).

But, a strange jitter effect would flash across the 24-inch's display, sporadically, every few seconds. It wasn't related to something the computer was doing at that moment, and the same display operates jitter-free when connected to a couple of different MacBook Pros here. Whether the problem is a bug that afflicts all new iMacs or just a quirk of the one we're testing, that we don't know yet. It's most likely the former, and most likely fixable through a firmware update, if previous Mac display quirks and how Apple has addressed them are any indication. If external display support is critical to you, though, you might want to hold off purchasing a 21.5-inch iMac for a bit.

The "Mid-2011" model of the iMac 21.5-inch recaptures the essence of the 2006 iMac 20-inch in all the right ways, though with the sort of display and performance improvements one would expect to have taken place in the years since then. Combine that with its array of connection options, including Thunderbolt, and the newest budget desktop Mac is a tremendous value, even when the lack of USB 3.0 and the potential hassle of future hard drive upgrades are factored in.

If your photography takes place mostly on location, then a desktop computer might not be for you. If you demand the very best possible display and processing speed, it might make more sense to go with an upper-end Mac Pro and a separate, premium-quality monitor. For every other Mac-based shooter, particularly those among you that like to get a lot but don't like to spend a lot, the new iMac 21.5-inch is an awesome photo workstation option.